The invention relates to an luminescent screen comprising an optical fibre plate formed by the reunion of elementary optical fibres constituted by bars of core glass surrounded by one or several cladding glasses, the optical fibre plate being provided with cavities disposed substantially perpendicularly to one of its faces, the cavities being obtained after having partially hollowed out one or several glasses, including the core glass, whilst preserving at least the cladding glass farthest remote from the core glass, the surfaces thus exposed constituting the walls of the cavities for the part substantially parallel to the axis of the elementary optical fibres, and the bottom of the cavities for the remaining part, the cavities being then filled by a luminescent product emitting a luminous flux under the influence of an electron bombardment.
It also relates to a display tube, for example an image intensifier tube used, for example in night-vision, or also a slit scanning tube, or also an X-ray converter tube, or even a cathode ray tube permitting the display of an image such as those used in oscilloscope or in television, or, generally, any tube permitting the display by means of the bombardment of a luminescent material by electrons.
A screen of this type is described in the article entitled "High Modulation Transfer Function (MTF) Phosphor Screens" by J. R. PIEDMONT and H. K. POLLEHN, published in SPIE, Vol. 99, Third European Electro-Optics Conference (1976), pp. 155-161. This screen is constituted by an optical fibre plate, each fibre being formed by a core glass surrounded by a cladding glass. In a prior art structure the luminescent material was deposited directly on the optical fibre plate and a lateral light dispersion is produced, limiting the contrast of the image and hence the performances of the system. Said authors also propose a screen comprising the above optical fibre plate, in which, however, the core glass has been removed down to a small depth in order to form the cavities. The walls of said cavities are formed by the cladding glass and the bottom is formed by the surface of the core glass previously attacked by a chemical solution which has selectively dissolved the core glass. Said cavities have a depth usually of the order of a few microns or a few tens of microns, the actual depth resulting from a compromise between the width of the cavities, the grain dimension of the luminescent material and the energy of the incident radiation. The inner walls of each cavity must be metallized in order to avoid the light emitted by a cavity from passing into the cavity of an adjacent fibre. Said cavities are then filled with a luminescent material. As for the conventional screens, an aluminium film is deposited on the grains of luminescent material, thus optically closing each cavity and electrically fixing the potential of the screen.
The screen thus formed is used in a display tube in which a flow of electrons will excite the luminescent material. Each cavity serves as a quasi-closed space and hence the light emitted in each cell can propagate only in the core glass.
Said diminution of the dispersion of the emitted light constitutes the essential advantage of the optical fibre screen. This leads to an improvement of the modulation transfer function which characterizes said dispersion. However, it appears from use that said improvement of the modulation transfer function is actually accompanied by a diminution of the light quantity restored at the other extremity of the optical fibre plate.
It is hence the object of the invention to improve the quantity of light restored at the other extremity of the optical fibre plate, while preserving the improvement of the modulation transfer function.